The Wash looks on the map like a large bite out of the coastline of eastern England. It separates the curved coast of East Anglia from Lincolnshire. It is formed by a large bay with three roughly straight sides meeting at right angles, each about 25 kilometres (15 miles) in length. All the eastern coast of the Wash is in Norfolk, and goes from Hunstanton in the north to the mouth of the River Great Ouse at King's Lynn in the south. The coast on the opposite side goes from Gibraltar Point to the mouth of the River Welland and is all in Lincolnshire. The southern coast goes roughly northwest-southeast, joining these two river mouths. In the middle is the mouth of a third river, the River Nene.

The most famous thing that people remember about the Wash is that "King John lost his crown in the Wash". This is because, according to reports from the time, King John travelled from Spalding in Lincolnshire to Bishop's Lynn, in Norfolk in the year 1216. He became ill and decided to go back. While he took the longer route through Wisbech, he sent his luggage, including his crown jewels, along the seashore and across the mouth of the Wellstream where the water was not deep. This route was only safe when the tide was low. The horse-drawn wagons moved too slowly for the incoming tide, and many were lost. The whole area at the time was an extensive wetland known as The Fens, which was later drained. It is close to sea level.